Magnetic tunnel junctions (MTJ) form promising candidates for nonvolatile memory storage cells to enable a dense, fast, nonvolatile magnetic random access memory (MRAM). The magnetic tunnel junction comprises at least two ferromagnetic layers separated by a thin insulating layer. The conductance of the device depends on the relative magnetic orientation of the magnetic moments of the ferromagnetic layers. The lateral size of the MTJ storage cell must be of sub-micron dimensions to be competitive with today's DRAM memories with 10-100 Mbit capacities. Moreover, the lateral size of the MTJ storage cell will need to be further reduced as memory capacities further increase in the future.
The required small size of the MTJ storage cell leads to several problems. First, as the lateral dimensions of the cell are reduced, the volume of each of the magnetic layers in the MTJ device is also reduced, which leads to the possibility of "super-paramagnetic" behavior, i.e., thermal fluctuations can cause the magnetic moment of a magnetic entity to spontaneously rotate if the magnetic anisotropy of the entity, which is proportional to its volume, is not sufficiently great. For example, a ferromagnetic thin layer with lateral dimensions of .about.0.3.times.0.3 (.mu.m).sup.2 and a thickness of 100 .ANG., is superparamagnetic at room temperature if comprised of permalloy, which has a very low magnetic anisotropy of .about.1000 erg/cm.sup.3. In practice, the effective magnetic anisotropy of the ferromagnetic layer would be considerably higher, even if comprised of a material with a small magnetocrystalline anisotropy, because small magnetic elements, unless circular, possess a magnetic shape anisotropy. Indeed, as the lateral dimensions of an MTJ element are reduced, the stray magnetic fields at the edges of the element, generated by the magnetic poles at the edges of the ferromagnetic layers of the MTJ, become of ever increasing importance. On the one hand these magnetostatic fields generate an effective in-plane magnetic anisotropy which will stabilize the magnetic structure against thermal fluctuations. However, these fields, which depend on the detailed shape of the MTJ cell, can become so large when the size of the MTJ device is reduced that the write fields themselves (and their associated currents through the corresponding write and bit lines) become so large that the power consumption of the MRAM becomes too high to be competitive. More importantly, these fields can lead to large magnetic interactions between the ferromagnetic layers within a single MTJ storage cell and between ferromagnetic layers in neighboring MTJ storage cells. In the first case, the state of the MTJ cell in which the magnetic moments of the free and fixed ferromagnetic layers are antiparallel will be much more energetically stable than the state in which the moments are parallel to one another. This leads to asymmetries in the magnetic switching between the parallel and antiparallel states of the MTJ cell, making it difficult to operate the MTJ cell. In the second case, the magnetic switching field of a given MTJ cell will depend on the magnetic state of its neighboring cells, which means that the margin of write operation of the memory array is reduced, eventually making it inoperable. Unless these magnetostatic interactions can be mitigated they will eventually limit the smallest size of the MTJ cells and thus the highest density of the MTJ MRAM.
MTJ memory storage cells can be comprised of various types of structures. In one type, a hard-soft (HS) MTJ, one of the ferromagnetic layers is designed to be magnetically "hard" (high coercivity) and have a very large magnetic switching field, while the other ferromagnetic layer is magnetically "soft" (low coercivity). The two memory states of the cell correspond to the soft layer having its magnetic moment oriented either parallel or antiparallel to the fixed moment of the hard layer. An improved HS MTJ is described in IBM's U.S. Pat. No. 5,801,984. In another type of MTJ, an exchange-biased (EB) MTJ, the magnetic moment of one of the ferromagnetic layers is fixed or pinned by exchange biasing it with an antiferromagnetic layer, and only the orientation of the magnetic moment of the free ferromagnetic layer is changed during read and write operations. An EB MTJ is described in IBM's U.S. Pat. No. 5,650,958.
For the EB MTJ the magnetostatic fields emanating from the exchange-biased pinned layer can be greatly reduced by replacing this layer with a sandwich of two ferromagnetic films antiferromagnetically coupled to one another by a thin antiferromagnetic coupling film, as described in IBM's U.S. Pat. No. 5,841,692. In this type of structure (also called an antiparallel pinned or AP pinned layer) the antiferromagnetic coupling film must be selected from a list of known materials and must be of a special thickness to give rise to the exchange coupling between the two ferromagnetic films in the sandwich. This AP pinned structure is based on the discovery of oscillatory coupling, as described in detail by Parkin et al. in "Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Cr and Fe/Cr", Phys. Rev. Lett., Vol. 64, p. 2034 (1990). The magnetic moments of the two ferromagnetic films in the AP pinned layer are aligned antiparallel to one another so that the net magnetic moment of the AP pinned layer is reduced compared to a pinned layer comprised of a single ferromagnetic layer. Since the strength of the magnetostatic field from a ferromagnetic layer is proportional to the net magnetic moment of the layer, the magnetostatic field from the AP pinned layer comprised of two antiferromagnetically coupled ferromagnetic layers is less than that of a pinned layer comprised of a single ferromagnetic layer.
It is also possible to reduce the strength of the magnetostatic fields emanating from the edges of the free ferromagnetic layer in a HS or EB MTJ by forming the free ferromagnetic layer from a sandwich of two antiferromagnetically coupled ferromagnetic films, as described in IBM's pending U.S. patent application Ser. No. 08/947,282 filed Oct. 7, 1997. However, this may have disadvantages. First, the antiferromagnetic coupling film is extremely thin so that the thermal stability of the antiferromagnetically coupled sandwich may not be adequate for the required lithographic and patterning processing steps to which the MTJ materials will be subjected. Second, the magnetic properties of the antiferromagnetically coupled sandwich may be inferior to that of the individual ferromagnetic films because of incomplete antiferromagnetic coupling of these films if the films are not, for example, extremely smooth. Also in an EB MTJ it is usually preferable to deposit the metallic antiferromagnetic layer and the pinned ferromagnetic layer to which it is exchange-biased first in order to achieve optimal exchange biasing. This is because metallic layers grown on top of the insulating tunnel barrier, which is usually formed from an amorphous layer of Al.sub.2 O.sub.3, may be rougher than the layers formed beneath the tunneling barrier. Typically metal layers do not "wet" oxide layers so that thin metal layers deposited on oxide layers are comprised of numerous islands of varying sizes and heights. Such metal layers are necessarily rough. Moreover, such layers will be poorly crystallographically textured. For these reasons, not only is it very difficult to prepare very thin ferromagnetic free layers with good magnetic properties but it may also be difficult to prepare antiferromagnetically coupled free layers with appropriate magnetic properties.
What is needed is a MTJ memory cell with an improved ferromagnetic free layer with reduced magnetostatic stray fields.